Field of the Invention
The present invention relates to a method of estimating colors for color image correction, which is suitable for the formation of an LUT (look-up table) in a color correcting apparatus of, e.g., a video printer or a digital color copying machine.
When hard copy of a TV image is to be produced by using, e.g., a color printer or a digital color copying machine, since each machine has its own color system, a color image correcting apparatus is used to match original colors with reproduced colors.
As is known, a color masking apparatus, for example, as one of color image correcting apparatuses is an apparatus for reproducing proper colors by canceling secondary absorbing components of a color material such as a toner or an ink.
Of TV images, color images are formed by a color addition method in which an RGB coordinate system of phosphors is used as its fundamental colorimetric system. In contrast to this, a color subtraction method is used to form color images on a photographic printing paper or the like. This method employs, e.g., a YMC fundamental color system. In such a case, conversion (color correction) of image data is performed between these fundamental color systems.
For example, as shown in FIG. 1, three primary color data of red R, green G, and blue B are converted into image data of yellow Y, magenta M, and cyan C by a color masking apparatus 10. These Y, M, C image data are supplied to a color printer 100.
If the color characteristics of a printer or the like can be accurately obtained, a combination of fundamental colors (e.g., Y, M, and C) which represents a target color can be obtained, thereby improving the color reproducibility.
A method of obtaining a combination of fundamental colors which represents a certain target color is disclosed in, e.g., Japanese Unexamined Patent Publication (Kokai) No. 63-254864. For the sake of a simple description, two fundamental colors (e.g., Y and M) are used.
FIG. 2 shows a YM coordinate system. Color patches are formed by supplying Y and M image data corresponding to the lattice points (5.times.5=25) of the YM coordinate system to a color printer.
Actual colors are measured from the color patch. The measurement values are then converted into values based on an L*u*v* colorimetric system by using a conversion formula. FIG. 3 shows a graphic pattern formed by plotting the converted values based on the L*u*v* colorimetric system. Vertices B, C, G, and F of a square in FIG. 2 respectively correspond to points B', C', G', and F' in FIG. 3.
A target value T' corresponding to an output color to be obtained is set in the L*u*v* colorimetric system. In this case, if the target value T' is set within an area defined by lattice points a' to d' as shown in FIG. 3, a combination of Y and M (target value T) in the YM coordinate system is estimated to be located in an area defined by lattice points a to d, as shown in FIG. 2.
The position of the target value T in the area defined by the lattice points a to d is obtained by performing convergence processing while mapping the colorimetric system in FIG. 3 on the coordinate system in FIG. 2. Convergence processing is performed in this manner for the following reason. Although conversion from the coordinate system in FIG. 2 into the colorimetric system in FIG. 3 is known, their inversion is very complicated, and any good conversion formula has not been known yet.
A specific one of areas defined by the 25 lattice points (see FIG. 3), in which the target value T' is present, is obtained. If the target value T' is present in an area S'O as shown in FIG. 5, it is estimated that the value is present in an area SO corresponding to the area S'O as shown in FIG. 4.
The estimated area SO is divided into four equal areas S1 to S4. Five division points e to i are calculated from weighting averages of surrounding lattice points. These division points e to i are then converted into values in the L*u*v* colorimetric system by using a conversion formula for the system. The converted values are plotted on the colorimetric system in FIG. 5, and a specific one of four areas S1' to S4' defined by plotted division points e' to i', in which the target value T' is present, is obtained. If the target value T' is present in the area S2' as shown in FIG. 5, it is estimated that the value is present in the area S2 corresponding to the area S2', as shown in FIG. 4.
The estimated area S2 is then divided into four equal areas S5 to S8. Five division points j to n are calculated from weighting averages of surrounding lattice points which have already been obtained. These division points j to n are then converted into values in the L*u*v* colorimetric system by using a conversion formula for the system. The converted values are plotted on the colorimetric system in FIG. 5, and a specific one of four areas S5' to S8' defined by plotted division points j' to n', in which the target value T' is present, is obtained. If the target value T' is present in the area S8' as shown in FIG. 5, it is estimated that the value is present in the area S8 corresponding to the area S8', as shown in FIG. 4.
The estimated area S8 is divided into four equal areas S9 to S12. Five division points o to s are calculated from weighting averages of surrounding lattice points which have already been obtained. These division points o to s are then converted into values in the L*u*v* colorimetric system by using a conversion formula for the system. The converted values are plotted on the colorimetric system in FIG. 5, and a specific one of four areas S9' to S12' defined by plotted division points o' to s', in which the target value T is present, is obtained. If the target value T is present in the area S10' as shown in FIG. 5, it is estimated that the value is present in the area S10 corresponding to the area S10', as shown in FIG. 4.
By repeatedly dividing areas in such a manner, matrices are gradually reduced in size and finally converge. By calculating an averge of four lattice points or division points of the area which converges, the target value T, i.e., a combination of fundamental colors representing an output color to be obtained can be obtained.
The above-described estimation operations are performed with respect to each target value T' in order to obtain a corresponding target value T.
The above-described color masking apparatus 10 may be designed such that, for example, a table is formed in advance on the basis of the target values T estimated in this manner, and a given target value T is referred to in accordance with input image data. In this case, R, G, and B image data are made to correspond to Y, M, and C image data through corresponding values of the L*u*v* colorimetric system.
In the conventional method of obtaining a target value T in this manner, since convergence processing is performed by repeatedly dividing areas, a long processing time is required.